1. Field of the Invention
The invention relates to human antibody IgG2 constant regions (Fc regions) mutated such that they substantially lose the capacity to specifically bind Fcγ receptors or activate mitogenic responses by immune cells by Fc receptor mediated cross-linking of surface target antigens. The invention also provides novel antibodies into which the mutated IgG2 constant regions can be incorporated.
2. Discussion of the Field
Antibodies that target cell surface antigens trigger unwanted immunostimulatory and effector functions associated with Fc receptor (FcR) engagement on immune cells and the activation of complement. As therapeutic antibodies and Fc-fusion constructs intended to target and activate or neutralize target ligand functions but not damage or destroy local cells or tissues that are needed, Fc mutants with ablated effector functions have been sought.
Human IgG isotypes (the subclasses of mature gamma globulin class G antibodies; IgG1, IgG2, IgG3 and IgG4) exhibit differential capacity to recruit immune functions, such as antibody-dependent cellular cytotoxicity (ADCC, e.g., IgG1 and IgG3), antibody-dependent cellular phagocytosis (ADCP, e.g., IgG1, IgG2, IgG3 and IgG4), and complement dependent cytotoxicity (CDC, e.g., IgG1, IgG3). Isotype-specific engagement of such immune functions is based on selectivity for Fc receptors on distinct immune cells and the ability to bind C1q and activate the assembly of a membrane attack complex (MAC). Among the various isotypes, relative affinity for Fcγ receptors (e.g., FcγRI, FcγRIIa/b/c, FcγRIIIa/b) is high for IgG1 and IgG3, however, there is minimal affinity for IgG2 (restricted to the FcγRIIa 131H polymorphism), and IgG4 only has measurable affinity for FcγRI. Using comparative sequence analysis and co-crystal structures, the key contact residues for receptor binding have been mapped to the amino acid residues spanning the lower hinge and CH2 region. Using standard protein engineering techniques, some success in enhancing or reducing the affinity of an antibody preparation for Fc receptors and the C1q component of complement has been achieved.
Among the isotypes, IgG2 is least capable of binding the family of Fc receptors. Using IgG2 as the starting point, efforts have been made to find a mutagen with diminished effector functions but which retains FcRn binding, prolonged stability, and low immunogenicity. Improved mutants of this nature may provide improved antibody therapeutics with retained safety.